1. Field of the Invention
The present invention relates to amplifiers, and more particularly to an amplifier which has its driving power supply circuit separated from the ground potential thereof.
2. Description of the Prior Art
In amplifiers for audio use, especially power amplifiers for output, various output types have been proposed.
FIG. 1 of the accompanying drawings is a circuit diagram showing an example of a BTL (balanced transformerless) amplifier.
The BTL amplifier is such that two sets of, e.g., SEPP (single-ended push-pull) amplifiers are operated in phase opposition to each other, and that a load is connected between the output points of the respective SEPP amplifers. This type is known as a system which enhances the voltage utilization factors of the SEPP amplifiers etc. still more.
In the arrangement of FIG. 1, a signal applied to an input terminal 1 is supplied to pre-amplifiers 2 and 3. The pre-amplifiers 2 and 3 are respectively constructed as a noninverting amplifier and an inverting amplifier. Outputs from the pre-amplifier 2 and 3 are respectively applied to SEPP output circuits 4 and 5 so as to drive a load, such as loudspeaker 8, which is connected between the respective output terminals 6 and 7 of the SEPP output circuits 4 and 5. On the other hand, a power transformer 9 (only a secondary coil is shown) which properly steps down an input voltage from a commercial power supply has its secondary side output rectified by a rectifier circuit 10 which is constructed of, e.g., a diode bridge. The center tap 9A of the power transformer 9 is connected to a reference potential E.sub.1. Smoothing capacitors 11 and 12 are respectively interposed between the center tap 9A and the outputs of the rectifier circuit 10 with positive and negative signs, to smooth both the positive and negative outputs of the rectifier circuit 10. Positive and negative D.C. voltages +B and -B thus produced are supplied to the output circuits 4 and 5 as supply voltages.
The respective pre-amplifiers 2 and 3 are provided with feedback circuits 2A and 3A so as to apply negative feedback.
The parts of the SEPP output circuits 4, 5 and the smoothing capacitors 11, 12 of the power supply in such circuit arrangement are extracted as shown in FIG. 2. In this figure, signal currents Y and Z flow through the transistors Tr.sub.1 -Tr.sub.4 of the SEPP output circuits 4, 5 and the load 8 as illustrated by arrows. That is, the smoothing capacitors 11, 12 of the voltage supply source are connected in series with the load 8. Here, letting C denote the capacitance of each of the smoothing capacitors 11 and 12, and R denote the resistance of the load 8, the time constant T between the power supply circuit and the load is given by the following equation: EQU T=1/2R C (1)
Since the supply voltages are +B and -B, the rated voltages of the respective smoothing capacitors 11 and 12 need to be the voltage B. Accordingly, the overall capacity P.sub.1 of this power supply circuit, which is, given by the product among the capacitance and rated voltage of each capacitor and the number of the capacitors, is expressed by the following equation: EQU P.sub.1 =C.times.B.times.2=2 C B (2)
In such arrangement, the two smoothing capacitors 11 and 12 are required for the positive and negative signs, and these capacitors are connected in series with the load 8. Regarding the time constant of the power supply circuit, therefore, the combined capacitance of the smoothing capacitors 11 and 12 becomes 1/2 as a whole. It turns out that smoothing capacitors of large capacitance are required for increasing the time constant of the power supply circuit. Moreover, the reference potential E.sub.1 for the great currents, which flow from the positive and negative supply voltage sources +B and -B to the load 8, a reference potential E.sub.2 for the input signal to be amplified, and reference potentials E.sub.3 and E.sub.4 for the respective pre-amplifiers 2 and 3 to amplify the input signal, are an identical potential (the points of the reference potentials are directly connected to one another). Therefore, the signal to be amplified flows, not only to the point of the reference potential E.sub.2, but also to the points of the reference potentials E.sub.3 and E.sub.4 of the respective pre-amplifiers 2 and 3. The hum etc. of the power supply accordingly have evil effects on the pre-amplifiers 2, 3 due to the great currents flowing through a point of the reference potential E.sub.1, and form causes for the degradation of the tone quality of an audio equipment provided with the amplifier.
Another prior-art power amplifier has an output arrangement as shown in FIG. 3. Referring to the figure, a differential amplifier circuit 13 is constructed of a pair of transistors Tr.sub.5 and Tr.sub.6, the base of one of which is supplied with an audio signal through an input terminal 1. The resulting output is applied to the base of a transistor Tr.sub.7 which constitutes an output power amplifier 14, and an amplified signal provided from the collector of this transistor Tr.sub.7 is applied to a loudspeaker, not shown, through an output terminal 16.
Numeral 17 indicates a current source. The differential amplifier circuit 13 and the output power amplifier 14 are supplied with positive and negative D.C. voltages +B and -B. These supply voltages are similar to those illustrated in FIG. 1, and are obtained in such a way that the secondary side output of a power transformer 9 is rectified by a diode bridge 10, whereupon the rectified outputs are smoothed by smoothing capacitors 11 and 12.
In such arrangement, the input and output operations of the audio signal are carried out by the unbalanced circuit, one line of which is held at the ground potential and connected to a chassis, a case or the like, along with one line of the power supply circuit. Accordingly, one input terminal is connected to a reference (ground) potential E.sub.2, one output terminal 16 is connected to a reference (ground) potential E.sub.6, one end of the differential amplifier circuit is connected to a reference (ground) potential E.sub.5, and the center tap 9A of the power transformer 9 and one terminal of each of the capacitors 11 and 12 are connected to a reference (ground) potential E.sub.1.
Such arrangement, however, has the disadvantage that when another equipment, e. g., a pre-amplifier is connected to the input terminal 1, a minute potential difference involved between the ground potentials of the amplifier and the equipment and also hum and other noise affect the ground potential of the amplifier, and they flow into the circuit of the audio signal, resulting in the degradation of the quality of the amplified signal. Another problem is that, since great currents flow from the power supply to the point of the ground potential E.sub.1, the hum component etc. of the power supply causes evil effects through the ground potential point E.sub.1 of the input terminal 1 and those E.sub.5 and E.sub.6 of the respective amplifier circuits 13 and 14.
A BTL amplifier having a single supply voltage as shown in FIG. 4 is also known which is so constructed as to avoid the overall capacity P.sub.1 of the power supply indicated by Equation (2), this capacity forming one disadvantage of the prior art as explained before. In this case, two rectifier diodes 10a and one smoothing capacitor 11a suffice. Since, however, the reference potential point E.sub.1 of the power supply and those E.sub.2, E.sub.3 and E.sub.4 of an input signal to be amplified are equipotential, inevitably the current of the power supply affects the signal adversely and degrades the tone quality of an audio equipment provided with the amplifier. Moreover, in such amplifier, a voltage substantially equal to 1/2 of the supply voltage B is applied between an output terminal 6 or 7 and the reference potential even during the absence of any signal. Therefore, it is dangerous to touch the output end and a chassis held at the reference potential.